A controlled atmosphere furnace melting system for forming ingots requires a means to withdraw a cast ingot from the furnace melting system. In the standard practice of ingot formation, a puller-head mold structure, such as a dovetail mold or a conventional threaded puller-head mold, is commonly used to withdraw a cast ingot. Often, puller-head mold structures are constructed with a channel, cavity, or slot to receive and capture the first casting of molten metal into the mold. That first casting into the channel, cavity, or slot serves to mechanically lock the initial portion of the overall semi-continuous casting onto or into the moveable bottom of the mold. This mechanical locking provides a location from which the casting can be pulled, and thus allows all subsequent cast and solidified material to be withdrawn from the mold, allowing room for more casting of molten metal which in turn is solidified and withdrawn, thereby forming an ingot. However, traditional puller-head mold structures present disadvantages when used for ingots having a relatively small diameter, or ingot formation of certain specialty or complex metal alloys.
Dovetail puller-heads can be constructed with two or more complementary or matching parts forming a channel, cavity, or slot, where the two or more complementary or matching parts can separate from around a cast ingot once the ingot has cooled. Slotted dovetail retention puller-heads, however, can sometimes fail under high tensile forces when there is a relatively low contact area between the ingot and dovetail puller-head structure, which can be limited to the area of the mechanically-locked portion of the casting. Removing the ingot with a dovetail puller-head structure can also require horizontal sliding of the ingot, pulling the ingot by the mechanically-locked portion of the casting, which exposes the ingot to mechanical forces that can cause galling, and thus can be particularly difficult to perform with long ingots. A further disadvantage is that molten material can also run out of the open end of the dovetail slot and cause binding of the ingot with the dovetail puller-head structure. Further, if the interface portion of the dovetail puller-head gets stuck in the middle of the withdrawal mold, then there is no way to remove the dovetail puller-head from the ingot without causing major damage to the mold and puller, and potentially damaging the ingot as well.
The construction of two-piece removable dovetails also have many drawbacks. Being constructed of separate pieces of material, the components of two-piece dovetail can suffer from poor heat transfer to the directly water cooled components of the withdrawal system. This can cause the dovetail to overheat or even melt. Further, using two-piece dovetails generally requires removing multiple small fasteners in order to remove the dovetails. This presents safety issues due to the operator having to work around the base of a potentially large, heavy, and extremely hot ingot. Moreover, such fasteners are generally steel components, which can overheat, melt, and/or become galled and brittle. Casting material can also run out of, around, and through the edge surfaces of the two separate pieces of the dovetail. Molten metal can also end up cast into, along, or in the spaces between, the edge surfaces of the two-piece structure, requiring that such casting be cut or ground out of the dovetail mold.
Conventional, basic threaded puller-head molds that include a female threaded hole in the puller-head into which molten material can be cast also suffer from problematic casting and formation issues. Such threaded puller-head molds generally have no relief, and the shrinkage of the cast metal upon cooling causes binding and galling along the interior wall of the mold. Known threaded puller-head molds are also generally limited in cross-section, which can lead to poor ingot to puller connection strength which can lead to breakage.
Accordingly, there remains a need for a puller head mold structure that can be used to withdraw a cast ingot from a furnace melting system without disadvantages known in the field.